EEG monitoring is a well known technique for determining brain function. The importance of continuous EEG monitoring during specific high-risk cerebrovascular processes is becoming widely acknowledged. It is recognized that EEG frequency and amplitude changes precede the development of neurological dysfunction and early therapeutic intervention can reverse this change or prevent further deterioration.
Heretofore, the monitoring of EEG signals with prior art EEG monitors has been an extremely cumbersome task requiring the use of large, bulky polygraphs and skilled technicians in order to interpret the EEG signals. The EEG signals are of a highly complex nature, involving a variety of signals such as alpha waves and beta waves, which are low in amplitude and vary rapidly with time. Rapid interpretation of the signals has therefore been extremely difficult, and appreciable time and operator experience is required for interpretation of this type of EEG output.
Some of the problems associated with the intraoperative utility of EEG monitoring, such as signal interpretation and bulky equipment, have been eliminated by a new generation of compact devices which display processed EEG data. Specifically, the PSA-1.TM. EEG monitor, described in U.S Pat. Nos. 4,412,547 and 4,424,816, and produced by Neurologics, Inc., assignee of the present invention, provides precise measurement of a patient's electrocerebral activity, without the need for large, bulky polygraphs and skilled technicians. The PSA-1.TM. EEG monitor provides easy to read displays for early detection of changes in brain function due to ischemia or hypotension, and the monitor's solid state circuitry provides reliable, instantaneous assessment of unit function and calibration.
As with other physiological monitoring systems, the reliability, precision and accuracy of an EEG monitoring system relies, at least in part, upon the precise and accurate placement of the electrodes on predetermined areas of the patient's head. For example, classical EEG procedure has, for many years, called for the placement of "reference" electrodes on the earlobes of a patient. Using the PSA-1.TM. EEG monitor, it is suggested that three electrodes be equally spaced laterally across the patient's forehead, in addition to the reference electrodes applied to the patient's earlobes. Typically, the electrodes are provided in the form of either needle electrodes or "stick-on" electrodes, and are applied individually to the predetermined areas of the patient's head.
This method of placement of the electrodes on the patient is many times a limiting factor in the reliability, accuracy and precision of the output of the EEG monitor. When variations in the absolute or relative locations of the electrodes occur, as for example when the two outer electrodes applied to the patient's forehead are misplaced with respect to the middle electrode, imprecise, inaccurate and/or unreliable results may be produced.
Furthermore, problems can arise if any of the five electrodes becomes dislodged or otherwise fails to maintain proper contact with the patient's skin. This problem is particularly acute when attaching the reference electrodes to the patient's earlobes, due to the irregular surface thereof. Because of the difficulties in firmly attaching an electrode to the earlobe, many EEG technicans instead attach the electrode over the mastoid region, behind the ear. The difficulty with using the mastoid region is that it is often contaminated by muscle artifact. Although muscle artifact is sometimes less a consideration in the operating room because of the use of muscular paralyzing agents, it is nevertheless advisable to reduce such artifact whenever possible. Further, in some patients the mastoid region can be an active electrical site, so that the EEG which is detected is not the same as would be obtained using the electrically neutral site of the earlobe.